Image tube



D. R. CARLO Dec. 11, 1956 IMAGE TUBE' Filed Dec. 21. 1954 'ISA /Nvf/vrog DONALD R. CARLO ATTORNEY IMAGE TUBE Donald R. Carlo, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation Application December 21, 1954, Serial No. 476,773

2 Claims. (Cl. 313-65) The present invention relates to image tubes, and more particularly to image tubes in which an electron image from a photocathode is electrostatically focused upon a uorescent screen.

One diiculty encountered in the operation of image tubes is that an electrostatic charge accumulates at random positions on the inner wall of the tube envelope in such places as to cause lateral deection of the electron beam, this lateral deflection being observed on the flucrescent screen as an offset or shifted image. It is necessary in some instances in which image tubes are used that the image on the uorescent screen be centered precisely for all conditions of tube operation.

An object of the invention is to provide an improved image tube in which the image on the screen is maintained in a given position for all conditions of tube operation.

Another object of this invention is to provide an image tube which makes use of an .external shield for compensating or nullifying the effects which envelope wall charges have on the electron beam.

To the accomplishment of the above and related objects, my invention may be embodied in the form illustrated in the accompanying drawing, attention being called to the fact, however, that the drawing is illustrative only, and that specific change may be made in the specific construction illustrated and described, so long as the scope of the appended claims is not violated.

The drawing is a longitudinal section of one embodiment of this invention.

Referring to the drawing, the tube envelope 1 of glass or the like material is preferably made in two sections of different diameters which are joined by an annular flange 2. This envelope 1 is tubular and carries in its left-hand end a photoelectric cathode 3 and in its righthand end a fluorescent phosphor screen 4. The cathode and screen elements as well as the other basic structure of this tube are conventional in the art as can be deter mined by reference to Day application Serial No. 291,825, entitled Tube Assembly, and tiled June 5, 1952, now Patent No. 2,708,250, and Flory et al. Patent No. 2,506,- 018. insofar as is necessary to obtain a complete understanding of this invention, these prior disclosures are incorporated herein.

Cathode sleeve 5 extends rearwardly from the cathode 3, and an accelerating anode sleeve 6 is coaxially supported to extend rearwardly from the sleeve 5. A suitable support, such as a metallic washer 6a, is used to secure the sleeve 6 to the wall of the envelope 1 as shown. Supported rearwardly and spaced from the accelerating anode 6 is an anode sleeve 7 which is conductively secured to the perimetral portions of the phosphor screen 4.

As will be seen from a close inspection of the drawing, a space X is provided between the two sleeves 6 and 7. in registry with this space X and providing a circumscribed enclosure is a conductive coating or shield 8 which is applied to the outer circumferential surface of the envelope 1. This coating extends into conductive contact with the supporting washer 6a and the anode sleeve 6, and in order to insure that the space X is completely enclosed the coating 8 is extended over the flange 2. This coating 8 may be composed of any suitable material, such as Aquadag, which is composed of conductive carbon carried by a liquid vehicle or any suitable metallic composition or foil. The

purpose of this conductive shield 8 is to provide a uniform wall of potential coaxially of the space X for a purpose which will be explained more fully hereinafter. If desired, an electrical insulating coating 9 may be applied over the shield 8, this insulating coating being composed of any suitable materal such as insulating varnish.

ln operation, suitable potentials are applied to the various electrodes as indicated in the drawing. A radiant energy image projected onto the cathode 3 produces an electron image which is accelerated axially of the tube by the various electrode elements 5, 6 and 7 to impinge the screen 4 which luminesces in accordance with the pattern of the electron image. As is wellknown, the elements 5, 6 and 7 provide an electron lens which properly focuses the electron image onto the screen 4.

It has been discovered that as the tube is switched on and oli wall charges accumulate on the inner wall of the envelope 1 at points adjacent to the metallic washer 6a. One such point is indicated by the shaded area 10. It is to be noted that these random charges do not extend uniformly around the entire tube wall, but instead are random as to position, area, and magnitude of charge. In operation, these random charges exert an electrostatic inuence on the electron beam as it traverses the space X between the two anode sleeves 6 and 7. From this it will appear as obvious that the position of beam impingement on the screen 4 is inueneed by this deflection, such that the reproduced irnage will be olf-center.

Of particular annoyance is the fact that these wall charges are not constant from one time of tube operation to another and will change in some instances almost every time the tube is turned on and off and then on again. Thus, as the tube is switched alternately on and ofi the image reproduced by the screen will appear to jump or shift to diferent lateral positions each time the tube is turned on.

This shifting condition is eliminated entirely by use of the shield 8 which provides a wall of potential around the exterior of the envelope 1 of such nature as will overcome the deecting efects of the random charges 10. Thus, -the tube with the additional electrode 8 circumscribing the space X is more reliable in operation, and is extremely simple to manufacture.

What is claimed is:

1. An electron image tube comprising a glass envelope of tubular shape, a photoelectric cathode in one end of said envelope capable of emitting an electron 'nuage in response to a radiant energy image, an anode screen in the other end of said envelope which emits light in response to electron impingement, at least one tubular electrode element in said envelope between said cathode and anode and being disposed coaxially with respect to the electron beam which extends from said cathode to said anode, said element having a metallic support which is secured to said envelope, said element being axially spaced from said anode thereby providing a space for said beam which is not surrounded by a tube electrode element, and a conductive shield on the outer circumferential surface of sa-id envelope in radial registry with the space between said element and said anode, said shield being conductively connected to said element and thereby providing a conductive tubular enclosure for 3 said space for compensating forany Wall charge on the tube envelope which may laterally deect said beam.

2. An electron image tube comprising a glass envelope of tubular shape, a photoelectrric cathode in one end;

of said envelope capable of emitting an electron image in response to a radiant energy image, an anode screen in the other end of said envelope which emits light in response to electron impingement, two accelerating anodes of tubular shape supported inside said envelope:V

on Vthe outer surface of Vsaid 4f y envelope encircling said space, the axial extent of said shield radially overlying said space, and a conductive connection betweenonc of said accelerating anodes and said shield whereby said shield provides a uniform wall of potential surrounding said space to prevent random wall charges on said cnvelope from deecting said beam as it traverses said space.

References Cited in the file of this patent UNITED STATES PATENTS 

